Most commercial water contains alkaline earth metal cations, such as calcium, magnesium, etc., and anions such as carbonate and oxalate. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and oxalate ion exceed the solubility of the calcium oxalate reaction product, a solid phase of calcium oxalate will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on the surfaces of a water carrying system, they form scale. Scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Calcium oxalate is a common substituent of scale which forms on the surfaces of apparatuses used for thermal treatment of aqueous solutions and suspensions. It is known that calcium oxalate scale can be removed by washing pipes, tubes, or other metallic surfaces upon which calcium oxalate deposits have formed with dilute aqueous acid solutions, such as hydrochloric acid or nitric acid solutions. However, due to the limited solubility of calcium oxalate in these acids, repeated washings are necessary. Thus, acid washing constitutes a costly and time-consuming operation. Acid washing also deteriorates equipment.
U.S. Pat. No. 4,108,680 discloses a process for removing calcium oxalate scale from metallic surfaces comprising contactng the scale with an aqueous suspension containing nitric acid and manganese dioxide at a temperature between 20.degree. C. and 95.degree. C. This process, like the acid washing process previously discussed, is directed to removal of calcium oxalate which has formed on metallic surfaces. These processes are not directed to prevention or inhibition of calcium oxalate precipitation.
U.S. Pat. No. 4,575,425 discloses the use of phosphate/polycarboxylate compositions to inhibit calcium oxalate precipitation. The instant invention represents an improvement over the method of the '425 patent.
Polyphosphates, such as a product which is 1.1 Na.sub.2 0:1.0P.sub.2 O.sub.5, can control calcium oxalate scales up to a pH of approximately 4. However, calcium oxalate scale is difficult to control as the pH increases above 4. Since acrylates control calcium oxylate precipitation at pH's above 6.0, the most critical range for control is a pH of about 4.0 through a pH of about 6.0. The inventors have discovered that polyacrylates having a weight average molecular weight ranging between 7,000 and 9,000 are especially effective (in combination with a phosphate) for controlling calcium oxalate scaling.
It is an object of this invention to provide an improved composition and process for the inhibition of calcium oxalate precipitation at pH's greater than or equal to 2.0, especially at pH's ranging from 4.0 to 6.0.
These and other objects of the instant invention are accomplished by a process in which an effective amount of an admixture comprising: (a) a compound selected from the group consisting of water soluble phosphates and (b) a polyacrylic acid or salt thereof having a weight average molecular weight of about 7,000 to about 9,000, preferably about 8,000, is added to an aqueous system, thereby preventing the deposition and precipitation of calcium oxalate scale on metallic surfaces in contact with the aqueous system over a wide pH range.